Chemistry Reference
In-Depth Information
O
N
N
O
N
NH
N
O
NH
OH
2
N
N
O
Cu
Cu
Cu
Cu
S
H
2
N
N
N
N
N
N
HN
S
O
O
N
N
N
-
Cu
O
N
-
tetrahedral
Type 1
('blue')
square planar
Type 2
OH-bridged dimer
Type 3
H
O
square-based pyramidal
(prion protein)
Figure 8.9
The three types of four-coordinate copper(II) centres found in most copper proteins. An example
of a rarer five-coordinate square-based pyramidal geometry of copper(II) found in the prion protein
appears at right.
colour than Cu
2
+
aq
ion, consistent with the usual observation that tetrahedral complexes
exhibit more intense absorbance bands than octahedral or square planar complexes, but are
similar in hue.
Type 2
has a 'non-blue' copper centre, with the copper coordinated to two or three nitrogen
donors in addition to oxygen donor(s) in a square planar geometry, with the different donor
set and shape together responsible for a markedly different colour compared with Type 1.
Type 3
has two copper centres closely adjacent, forming a hydroxide-bridged dimer with
each copper ion in an approximately square planar geometry.
In this family of compounds, the N-donors come from unsaturated nitrogens in histidine
amino acid residues, the S-donors come from methionine and cysteine residues, and the
O-donors come from a carboxylic acid in the protein chain. Water, hydroxide and alkoxide
oxygens are also employed as O-donor ligands.
Electron transfer copper proteins
usually belong to the blue copper proteins (
Type 1
);
azurin
is a simple example. This family of proteins are also called
cupredoxins
, and they
participate in many redox reactions involved in processes fundamental to biology, such
as respiration or photosynthesis. The striking electron transfer capabilities of blue copper
proteins have been studied extensively.
Plastocyanin
, with a tetrahedral CuN
2
S
2
core, acts
as the electron donor to
Photo System I
in photosynthesis in higher plants and some algae.
Non-blue copper centres are actually most common in copper proteins, and the copper
centre in these adopts, as mentioned above, essentially square-planar geometry. The copper
ion is bound to the imidazole nitrogen of two or three histidine residues and to O-donor
ligands; there is weak additional O-donor coordination in axial sites, with the typical
Jahn-Teller distortion expected of d
9
Cu(II) complexes. The
Type 2
sites are more ionic
than
Type 1
sites, having mainly neutral donor groups rather than the thiolate anions of
the latter. Working cooperatively with organic coenzymes,
Type 2
copper centres direct a
wide range of biological oxidation reactions, which include alcohol oxidation and amine
degradation. In
caeruloplasmin
,a
Type 2
monomer joins with a
Type 3
dimer to form a
larger copper trimer; similar clusters appear in
laccase
and
ascorbate oxidase
.
Compounds with another stereochemistry are also observed. The copper binding site in
the prion protein, an approximately 200-amino-acid residue glycosylated protein that carries
one copper ion, is of a square-based pyramidal form (Figure 8.9), having one imidazole
nitrogen, two amide nitrogens and an amide oxygen bound around the copper, with a water
